Skeletal Muscle Fibers: Types and Functions

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  • 0:07 Muscle Fiber Types
  • 2:55 Fast Fibers
  • 5:00 Slow Fibers
  • 6:45 Intermediate Fibers
  • 8:06 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Did you know most human muscles contain a mixture of fast, slow and intermediate fibers? This lesson describes the structure and function of the three muscle fiber types found in humans.

Muscle Fiber Types

I enjoy a good BBQ on a warm summer day; what about you? Do you prefer light or dark meat? I prefer the light meat myself. You see, I had a bad experience with dark meat when I was a kid. I bit into an undercooked chicken leg, and when I pulled the meat apart I saw the blood vessels, and that kind of grossed me out. While this may seem nasty, it is nonetheless meaningful for the purpose of our discussion. The dark meat of a chicken leg is red because it contains a large blood supply. We'll talk about the significance of this in a bit. Light meat, on the other hand, is more white then red because it contains very little blood supply.

So, what does all this discussion about chicken have to do with muscle fiber types and muscular contraction? Dark meat is composed primarily of slow fibers, and these slow fibers are more dependent on blood supply and oxygen, whereas light meat is composed of fast fibers, which are less dependent on blood and oxygen. There's a third fiber type, and we refer to them as intermediate fibers. They have features that are in between fast and slow fibers. Regardless of the fiber type, muscle performance can be described in terms of force - that is, the maximum contraction strength of the muscle - and endurance, the amount of time the muscle can maintain a contraction once stimulated.

Most human muscles, in contrast with those of chickens, contain a mixture of the fiber types and thus appear pink. The relative distribution of fiber type within a muscle is genetically determined, but it can be influenced by exercise. In this lesson, we will describe the structure and function of the three different types of muscle fibers in terms of force and endurance. Before we discuss the different fiber types, let's define a few terms. First of all, aerobic metabolism refers to the synthesis of ATP requiring oxygen, whereas anaerobic metabolism, also referred to as glycolysis, refers to the synthesis of ATP not requiring oxygen. What about ATP? ATP is the energy source that's needed for muscular contraction within the cells. Now that we know some of the important terms, let's figure out how the muscle fibers work.

Fast Fibers

The majority of human muscle fibers are fast fibers, also referred to as fast-twitch glycolytic fibers, which, as the name suggests, contract quickly and depend on glycolysis for their source of energy. The eye muscles contain fast fibers and are thus capable of very rapid movement. These fibers are designed for rapid and strong contractions that don't need to last very long. Overall, fast fibers have high force and low endurance. Think of the chicken flapping its wings. In order to fly, the wing muscles have to contract and relax very quickly. These are fast fibers. A typical fast-twitch fiber will reach a peak contraction in about 0.01 seconds - now, that's pretty fast! However, the chicken can flap its wings for only a short period of time before the fast fibers fatigue.

Let's look at these fast fibers some more. Fast fibers are packed with myofibrils, and that gives them a large cross-sectional area and makes them very strong. The weakness of these fibers, as we've said before, is they fatigue quickly. They contract rapidly, and that uses up a lot of ATP in a short period of time. Furthermore, they contain little mitochondria to support aerobic synthesis of ATP. As such, these fast fibers are primarily dependent on anaerobic synthesis of ATP and are sometimes referred to as anaerobic, or even glycolytic, fibers. To help make up for the lack of mitochondria, fast fibers have a large glycogen store, and that provides the fibers with glucose for anaerobic metabolism.

Slow Fibers

Slow fibers, a.k.a. slow-twitch oxidative muscle, as the name suggests, contract slowly, and they depend on aerobic metabolism for energy. These fibers take three times as long as fast fibers to reach peak contraction, hence the term 'slow twitch.' Slow fibers are specialized to maintain prolonged contraction, as we need for standing or sitting. Human postural muscles, along with the chicken leg, contain a lot of slow fibers. These slow fibers have relatively low force, or contraction strength, and high endurance.

Slow fibers contain lots of mitochondria. They are surrounded by a large capillary network that provides the oxygen needed to support aerobic metabolism and are thus referred to as aerobic, or oxidative, fibers. Slow fibers also contain myoglobin, an oxygen-carrying pigment similar to hemoglobin in the blood. With more blood supply, more mitochondria and more myoglobin, slow fibers can produce more ATP during the prolonged contraction. Furthermore, as they contract slower, they don't require as many contractions to maintain a prolonged contraction; therefore, they don't need as much ATP. Finally, slow fibers are more efficient in terms of energy utilization, as they can metabolize fats in addition to sugar to make ATP.

Slow fibers are surrounded by oxygen-supplying capillaries.
Slow Fibers

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